Antibodies to various isolated histone classes demonstrate the association of histones with transcriptionally active nonribosomal genes in embryos of Drosophila melanogaster and the depletion of histones in the puffed regions of the polytene chromosomes of Chironomous thummi. The staining of histones in puffs is markedly reduced when compared to adjacent bands or interbands. Newly replicated chromatin is associated with an unaltered complement of histones as detected immunologically. Concanavalin A, a plant lectin, binds specifically to chromatin from rat liver with a high association constant. Electrophoresis followed by detection of proteins by their binding of the lectin reveals that a limited number of the nonhistone proteins have sugar residues bound by concanavalin A. The lectin thus might serve as a nondisruptive probe for the chromosomal glycoproteins. A probe different in kind, triiodothyronine, also binds specifically and with high affinity to liver chromatin. In vivo experiments suggest a physiologic relevance for the class of binding sites under study. Using the binding assay previously described, certain additional features of the interaction have been discerned; of particularly interest is an apparent cooperativity for the interactions of the hormone with chromatin. BIBLIOGRAPHIC REFERENCES: Bustin, M. (1977) Histone antibodies and chromatin structure. ICN-UCLA Symposium on Molecular Human Cytogenetics, in press. Bustin, M., Kurth, P.D., Moudrianakis, E.N., Goldblatt, D., Sperling, R. and Rizzo, W. (1977) Immunological probes for chromatin structure. Cold Spring Harbor Symp. Quant. Biol., in press.